The use of a pressurized flow of abrasives to clean a surface has been common for many years. In the past, two types of devices have been used to accelerate the abrasive against a surface. One type of apparatus feeds the abrasive into a high speed rotating wheel similar to a centrifugal fan. As the abrasive slides along the vanes of the wheel, it is accelerated and propelled against the object to be cleaned.
The second type of device uses compressed gas, normally air, to transport the abrasive through pipes and hoses. In this type of device, the compressed air used to transport the abrasive is expanded through a nozzle at the end of a pipe or hose and accelerates the abrasive against the object to be cleaned. In some devices, additional air is fed into the nozzle to assist in accelerating the abrasive.
Within the last ten years or so, devices which use water as a cleaning agent have been introduced in one form or another. Loosely termed "hydroblasters", these devices include units which use only water under very high pressure and also units which use water in connection with an abrasive. This application focuses on the second type of hydroblaster which uses a liquid propellant and an abrasive to clean the surface of an object.
An example of this second type of hydroblaster is shown in U.S. Pat. No. 4,218,855 to Wemmer which discloses a particulate spray nozzle for use with water and sand. As is believed to be common of most watersand hydroblasters, Wemmer uses the flow of the water to create a vacuum to draw the abrasive into the flow and out against the surface. In particular, Wemmer states that the "liquid pressure nozzle disclosed cooperates with the discharge bore of the blasting nozzle to create a full vacuum in the mixing chamber while at the same time maximizing impact velocity of the propellant and particulate mixture." Column 2, lines 23-28. It is believed that the devices similar to Wemmer which depend upon the creation of an adequate vacuum to cause particulate flow, however, are dependent upon very precise control of the spread of the liquid spray and the shape of the discharge bore to create and theoretically maintain the vacuum. If pressures of several thousand PSI are used, however, very slight changes in the shape of the orifice as well as changes in the discharge bore will cause the vacuum in the mixing chamber to change and thus also change the abrasive flow per unit of time. Because of the tendency of water under pressure to cause such erosive changes, personal experience has shown that abrasive flow control dependent upon a vacuum is difficult at best.
Even assuming that a constant vacuum is possible, such vacuum still may not be adequate for some purposes. Wemmer states that the interrelationship between the discharge bore and the spray stream may produce a full vacuum of up to and exceeding thirty (30) inches of mercury relative to ambient pressure. Column 3, lines 9-12. It is believed that this differential pressure is insufficient to consistently transport many materials whose particle sizes are desirable as good abrasives (16 to 40 mesh, materials). It is therefore the present inventor's experience that the utilization of a vacuum to cause abrasive flow has proven to be inadequate for many cleaning applications.
Attempts to provide a suitable nozzle which induces abrasive flow by other than vacuum force, however, have encountered numerous problems. The greatest problem is the rapid destruction of nozzles caused by improperly feeding the abrasive into the liquid propellant stream. Another problem is caused by wetting of the abrasive such that it clogs the nozzle.
It is therefore an object of the present invention to provide a new and improved system for use in treating a surface by propelling an abrasive against the surface with a liquid propellant which induces abrasive flow for the larger particle size abrasives and which will not rapidly destroy or clog the nozzle.